Apparatus for suppression of ion feedback in electron multipliers



March 19, 1968 G. w. GOODRICH 3,374,380

APPARATUS FOR SUPPRESSION OF ION FEEDBACK IN ELECTRON MULTIPLIERS FiledNov. 10, 1965 I 7 z 42 7 Kg n Va li C VARIABLE VOLTAGE "\"15 SOURCE 545s INVENTOR.

GEORGE w. GOODRICH ATTORNEY United States Patent Office 3,374,380Patented Mar. 19, 1968 ABSTRACT OF THE DISCLOSURE A pair of channelelectron multipliers operating in series and disposed at an angle tominimize ion feedback.

This invention pertains to apparatus for suppression of feedback inelectron multipliers and particularly those multipliers having arelatively unobstructed linear multiplying path.

In the prior art electron multipliers wherein a multiplying path isdefined by a linear tube with secondary emissive resistive coatingapplied to the interior of the tube and a potential gradient appliedacross the ends of the tube, a problem has been experienced which iscaused by ion or other type feedback in the tube. Such a multiplier isfrequently used with a photocathode or other source of electrons whichare to be multiplied. Positive ions are produced by the electronsstriking residual gas molecules near the output end of the tube wherethe electron concentration is particularly high and these positive ionsare accelerated toward the photocathode and either strike thephotocathode or a portion of the tube near the photocathode producingspurious electrons which are multiplied along substantially the entirelength of the multiplier. The presence of these spurious electrons canresult in limited gain, added noise, or reduced sensitivity of thephotocathode surface.

This invention overcomes these problems by providing apparatus togreatly reduce the effects of the ion feedback and the spuriouselectrons. This reduction is accomplished by forming the multiplier tubeinto at least two sections which may be called an input section and anoutput section which have nonlinear longitudinal axes so that the ionsformed in the output section which are accelerated in a reversedirection will impact into the walls of the input section in an areajust preceding the output section. This greatly reduces the number ofelectrons caused by the positive ion since the nearer the impact is tothe output end, the fewer electrons will be produced that have aspurious origin.

The multiple sections of the multiplier tube can have separate potentialgradients placed thereacross and can have surfaces of differentconductivities. By making the conductivity of the output section higherthan that of the input section, the high power dissipation is limited toonly that section where the amplified signal is also at a high level.

Also, a switch may be placed between the input section and outputsection to switch off the multiplier. Further, three or more sectionsmay be used with the longitudinal axes of adjacent sections beingnon-linear.

These and other objects will become more apparent when the followingpreferred embodiments are considered in connection with the drawings inwhich:

FIGURE 1 is a partly broken away elevational View of an imageintensifier embodying this invention which has a large number ofmultiplier tubes in each of two multiplier arrays;

FIGURE 2 is a section taken at 22 of FIGURE 1 showing the angle made bythe longitudinal axes of the multiplier tubes in adjacent arrays;

FIGURE 3 is a greatly enlarged view showing an electron multiplier ofthis invention having an input multiplier section and an outputmultiplier section with a photocathode at the input and a phosphorscreen at the output;

FIGURE 4 is a greatly enlarged View of a portion of an array ofmultiplier tubes showing the field existing in the tubes and at the tubeends; and

FIGURE 5 is a cross-sectional view of another embodiment of thisinvention showing a multiplier having three multiplier sections andbeing joined at the section interfaces.

Looking at FIGURES 1 and 2, an image intensifier may be seen having aphotocathode 20, input multiplying array 22, output multiplying array24, and phosphor screen 26. Potential sources 28, 30, 32 and 34 provideincreasing potential between photocathode 2t array 22, array .24 andphosphor screen 26. Source 35 provides during operation of theintensifier a zero voltage drop or a voltage that makes array 24slightly more positive than array 22, thereby providing for electrontravel from array 22 to array 24. If it is desired to interruptoperation, a potential making the input of array 24 more negative thanthe output of array 22 by an amount such as volts is applied by source35 thereby preventing an electron signal from passing from array 22 toarray 24.

Photocathode 20 may be made of materials known to the art such asantimony, cesium, potassium or sodium and phorphor screen 26 may also bemade of materials known to the art such as activated sulfide of Zinc andcadmium. In this preferred embodiment, arrays 22 and 24 have anextremely large number of channels 23 having a length to diameter ratioof about 25 to 1 and having a small diameter. These channels 23 have asecondary emissive resistive coating on the inside thereof.

Array 22 has conductive coatings 36 and 38 deposited or otherwise coatedon respectively the input and output surfaces and array 24 has coatings40, 42 deposited or otherwise coated on respectively its input andoutput surfaces. The conductive coating may be made of chromium,platinum, gold, or silver. The purpose of the conductive coatings is toplace the same voltage drop across each tube or channel 23 in array 22and to place the same voltage drop across each channel 25 in array 24.

The tubes 23, 25 in arrays 22 and 24 form an included angle of 166 inthis embodiment, shown in FIGURE 3. which has been found suitable toreduce ion feedback when the tubes 23, 25 have an overall length todiameter ratio of 50 to 1. Arrays 22 and 24 may be fabricated by placinga large number of small diameter glass tubes in a jig and heating untilthe tubes become fused to one another but before they'become distortedto form a billet. Arrays having the desired channel inclination can bemade by appropriate angular cutting from this billet. By utilizingsufficient compounds of lead such as lead oxide in the glass, thesecondary emissive resistive surface may be formed by passing hydrogengas for 816 hours through the tubes while they are heated to atemper-ature of 300-500 centigrade. Silicate glass having about 30percent of PhD on a molecular basis will produce a secondary emissiveresistive surface.

The advantages of incliningthe channels relative one another will now beconsidered in more detail. in multiplying tubes of the prior art,as-illustrated in the patent to Goodrich and Wiley, Patent 3,128,408issued Apr. 7, 1964, and entitled, Electron Multiplier, the input isdisposed to receive incoming particles which cause secondary emission ofelectrons after striking the surface of the tube. These electrons arefurther caused to strike the walls of the tube resulting in additionalsecondary emission so that a forward progressing, ever-increasingavalanche of secondary electrons is produced. These secondary electronsbecome extremely numerous near the output end of the multiplier tube andthe probability of an electron interacting with a residual gas moleculein the tube becomes relatively high, even if the residual gas pressureis very low.

The interaction of an electron with a residual gas molecule results indislodging an electron from the molecule giving the molecule a positivecharge so that it becomes a positive ion, indicated at 7 in FIGURE 3.The ion being opposite in sign to the electrons is caused to travelalong the tube in a direction opposite to the electron travel directionand is likely to be accelerated in a straight path for nearly the entirelength of the channel and impact against either the photocathode or aportion of the tube near the photocathode. The reason the ion is likelyto travel in a direction which is nearly parallel to the axis of thetube is that its initial velocity transverse to the axis of the tube isvery small and will be only on the order of thermal velocity andtherefore about of an electron volt at room temperature. This energy issubstantially less than the average energy of emission of an electron.The velocity energy imparted to the ion by the electric field within themultiplier is considerably higher and therefore the longitudinaldistance traveled by the ion down the tube will be much greater than thetransverse distance traveled. The ion path is shown at 72 in FIGURE 3and due to the included angle between multiplier sections 23, 25, theion strikes the section 23 at 74, near the output end of the tube, andhence, the amount of electrons produced by ion 70 will be much less thanif ion 70 impacted against photocathode 20 or near the input of tube 23.

Arrays 22 and 24 preferably are separated from each other by a gap whichis in the order of the channel tube diameter, in order to minimize Moirepattern beating effects. This spacing may be achieved by placing spacersbetween the arrays and then clamping, soldering or brazing the arraystogether. Insulative spacers are shown in the embodimentof FIGURES l and2 while a conductive spacer is shown for the embodiment of FIGURE 3.

It is not necessary that the channels 23, 25 at the array interface beperfectly aligned. Good resolution is obtainable if the channel diameteris kept reasonably small. The spread of electron signal from the outputof the channels in the array 22 is proportional to the diameter of thechannels in array 22 and by minimizing the channel diameter, the spreadof the electron signal is minimized.

The included angle of the arrays may be increased, the limitation beingto prevent ions formed in the output array 24 from being able to pass ina straight line to the photocathode 20 or to an area of the channels ininput array 36 which is near photocathode 20. Preferably, the includedangle should be small enough to prevent an ion from taking a linear pathof about more than 1.5 channel section lengths before impacting on achannel wall. As explained, by limiting the ion impact to the outputportion of array 22, the effect of the feedback ions is minimized. Also,it is desirable not to make the angle too small since the individualmultiplier channel openings become highly oval and this imparts poorresolution to the tubes.

The electric field existing in an array is shown in FIG- URE 4 wherefield lines 44 are shown parallel to the walls of the channel in thearray but bend upon leaving the array channels to assume a directionthat is substantially perpendicular to the unipotential surfaces 36 and38 in the case of array 22.

The number of array sections included in a multiplier assembly willgenerally increase as the required gain of the multiplier assemblyincreases. The two section multiplier shown in FIGURES 1 and 2 hasprovided a gain 10 to with voltages of 800 volts being placed acrosseach of arrays 22, 24 respectively. Higher gains may be realized byusing the three section multiplier of FIGURE 5 which has photocathode50, phosphor screen 52, potential 4 sources 54, 56, 58 and multiplier60, which has sections 62, 64 and 66. Sections 62, 64 and 66 may beconstructed in the manner of sections 22 and 24 with the channel axes inadjacent sections being non-linear so that the multiplying path changesdirection several times between photocathode 50 and phosphor screen 52.Also the device of FIGURE 5 has only one potential source 56 across allthree multiplier sections 62, 64 and 66. The arrays 62, 64 and 66 arepreferably spaced by conductive material which has a dimension in theorder of a channel diameter and since the spacing material isconductive, one battery may be used for all three sections.

Although this invention has been disclosed and illus- -trated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. For example, light feedback is likewiseprevented by this invention. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims. Having thusdescribed my invention, I claim: 1. Apparatus comprising a plurality ofelectron multipliers each having an input end for receiving particles tobe multiplied and having an output end for discharging electrons whichare in a multiplied ratio of the received particles and having alongitudinal axis between said input and output ends, each of saidelectron multipliers having a substantially continuous resistive surfacefrom its input end to its output end, against which said receivedparticles impact to produce secondary emission of electrons and againstwhich said secondary emission electrons impact to produce furthersecondary emission electrons thereby resulting in an avalanche ofelectrons, at least a first and a second of said multipliers beingplaced in series so that the output end of said first multipliersupplies the electron input of said second multiplier, particle emittingmeans at the input end of said first multiplier, voltage means connectedacross said multipliers to establish a longitudinal electrical currentflow in said resistive surface and to establish a substantiallylongitudinal field with substantially no transverse field component foraccelerating the electrons of secondary emission from the input of saidfirst multiplier towards the output of said second multiplier, with theconcentration of the avalanching electrons in said two multipliers beinghighest at the output end of said second multiplier, and with theprobability of said accelerated electrons ionizing any residual gasmolecules in the multipliers being higher at the output end of saidsecond multiplier, and means for minimizing feedback output caused byionization of the gas molecules in said second multiplier comprisingmeans for supporting said first and second multipliers with thelongitudinal axes thereof disposed to intersect at an angle forpreventing substantially all of the ionized gas molecules in said secondmultiplier from passing completely through said first multiplier and forcausing those ionized gas molecules to strike the surface of said firstmultiplier towards the output end of said first multiplier. 2. Theapparatus of claim 1 with said first and second multipliers comprisingindividual tubes having longitudinal dimensions substantially largerthan transverse dimensions, the output end of said first multiplierbeing spaced from the input end of said second multiplier by a distancesubstantially equal to the transverse dimension to minimize Moirebeating effect. 3. The apparatus of claim 1 with said first multipliercomprising a multitude of individual multiplying channels having theirlongitudinal axes parallel and joined together to form a firstmultiplier array,

said second multiplier comprising a multitude of individual multiplyingchannels having their longitudinal axes parallel and joined together toform a second multiplier array.

4. The apparatus of claim 3 with photocathode means being placedadjacent the input end of said first multiplier array,

phosphor means being placed adjacent the output end of said secondmultiplier array.

5. The apparatus of claim 3 with a first set of voltage leads from saidvoltage means being placed across said first array and a second set ofvoltage leads from said voltage means being placed across said secondarray.

6. The apparatus of claim 1 with means being connected to said first andsecond multipliers for switching on and oil the electron stream.

7. The apparatus of claim 1 with the included angle between saidlongitudinal axes being approximately 166 and the overall length todiameter ratio of said multipliers being on the order of 50:1.

8. The apparatus of claim 1 with at least three multipliers in end toend relation with the longitudinal axes of adjacent multipliers beingnonlinear so that gas molecules ionized in any of at least certain ofsaid multipliers can not travel in a reverse direction more than one andone half multiplier lengths before impacting against a multiplier wall.

9'. The apparatus of claim 1 with at least three multipliers in end toend relation with the longitudinal axes of adjacent multipliers beingnonlinear to minimize effects of ion feedbacks.

10. Apparatus comprising a plurality of electron multipliers each havingan input end for receiving particles to be multiplied and having anoutput end for discharging electrons which are in a multiplied ratio ofthe received particles and having a longitudinal axis between said inputand output ends,

each of said electron multipliers having a continuous substantiallyresistive surface from its input end to its output end, against whichsaid received particles impact to produce secondary emission ofelectrons and against which said secondary emission electrons impact toproduce further secondary emission electrons thereby resulting in anavalanche of electrons,

at least a first and a second of said multipliers being placed in seriesso that the output end of said first multiplier supplies the electroninput of said second multiplier,

particle emitting means at the input end of said first multiplier,

voltage means connected across said multipliers to establish alongitudinal electrical current flow in said resistive surface and toestablish a substantially longitudinal field with substantially notransverse field component for accelerating the electrons of secondaryemission from the input of said first multiplier towards the output ofsaid second multiplier, with the concentration of the avalanchingelectrons in said two multipliers being highest at the output end ofsaid second multiplier, and with the probability of said acceleratedelectrons ionizing any residual gas molecules in the multipliers beinghigher at the output end of said second multiplier,

and means for minimizing feedback output caused by ionization of the gasmolecules in said second multiplier comprising means for supporting saidfirst and second multipliers with the longitudinal axes thereof disposedto intersect at an angle for preventing substantially all of the ionizedgas molecules in said second multiplier from passing completely throughsaid first multiplier and for causing those ionizing gas molecules tostrike the surface of said first multiplier towards the output end ofsaid first multiplier,

said first multiplier comprising a multitude of individual multiplyingchannels having their longitudinal axes parallel and joined together toform a first multiplier array.

said second multiplier comprising a multitude of individual multiplyingchannels having their longitudinal axes parallel and joined together toform a second multiplier array, and

a first set of voltage leads from said voltage means being placed acrosssaid first array and a second set of voltage leads from said voltagemeans being placed across said second array,

the conductivity of the resistive surfaces of said second array beinggreater than the conductivity of the resistive surfaces of said firstarray.

References Cited UNITED STATES PATENTS 2,872,721 2/1959 McGee 3l3-105 X3,128,408 4/ 1964 Goodrich et al. 3l3103 X 3,197,663 7/1965 Norman etal. 313103 3,240,931 3/1966 Wiley et a1. 313-103 X JAMES W. LAWRENCE,Primary Examiner.

ROBERT SEGAL, Examiner. P. C. DEMEO, Assistant Examiner.

